C. F. Richter

Depth and geographical distribution of deep-focus earthquakes

Early in the history of seismology it was occasionally suggested that, in addition to earthquakes with foci comparatively near the surface of the earth, shocks might also originate at depths of the order of several hundred kilometers. However, down to a comparatively recent date, all such conclusions were either purely speculative or were based on inadequate or misinterpreted data.

SEISMICITY OF THE EARTH (SUPPLEMENTARY PAPER)

This paper supplements Seismicity of the earth (Gutenberg and Richter, 1941). Additional epicenters for shallow and deep earthquakes are reported. Sixty-four great earthquakes are now identified for 1904-1943, and 201 major earthquakes for 1922-1943. New maps are given. Relative seismicity of active regions is discussed quantitatively (shallow shocks only). The Pacific belt has about 80 per cent and the trans-Asiatic zone about 10 per cent of the general activity. Earthquakes, volcanism, and gravity anomalies are discussed in their geographical and dynamical relation to structural arcs of Pacific type. These must be maintained by persistent processes in which subcrustal currents play a part.

Depth and geographical distribution of deep-focus earthquakes (second paper)

This paper is a continuation of a previous publication (1938). The writers distinguish (1) shallow shocks, at depths not exceeding about 50 kilometers; (2) intermediate shocks, at depths from about 50 to 300 kilometers; (3) deep shocks. Separate maps are drawn for intermediate and for deep shocks, and new or revised determinations are presented and discussed. Previous conclusions as to mechanism and origin remain unmodified.

Possible Seismicity of the Nacimiento Fault, California

Earthquake history and instrumentally located epicenters suggest that this fault is active at depth.